R and S Absolute Nomenclature (Abhishek Mourya Sir) PATANG IIT JEE Forum

Rand S can be used to describe the configuration of a chiral centre

Before going on to talk about single enantiomers of chiral molecules in more detail, we need to
explain how chemists explain which enantiomer they’re talking about. We can, of course, just draw a
diagram, showing which groups go into the plane of the paper and which groups come out of the
plane of the paper. This is best for complicated molecules. Alternatively, we can use the following set
of rules to assign a letter, R or S, to describe the configuration of groups at a chiral centre in the
molecule.

Here again is the enantiomer of alanine you get if you extract alanine from living things.

1 Assign a priority number to each substituent at the chiral centre. Atoms with higher atomic
numbers get higher priority.

Alanine’s chiral centre carries one N atom (atomic number 7), two C atoms (atomic number 6),
and one H atom (atomic number 1). So, we assign priority 1 to the NH2 group, because N has the
highest atomic number. Priorities 2 and 3 will be assigned to the CO2H and the CH3 groups, and
priority 4 to the hydrogen atom; but we need a way of deciding which of CO2H and CH3 takes
priority over the other. If two (or more) of the atoms attached to the chiral centre are identical,
then we assign priorities to these two by assessing the atoms attached to those atoms. In this case,
one of the carbon atoms carries oxygen atoms (atomic number 8), and one carries only hydrogen
atoms (atomic number 1). So CO2H is higher priority that CH3; in other words, CO2H gets
priority 2 and CH3 priority 3.

2 Arrange the molecule so that the lowest priority substituent is pointing away from you.
In our example, naturally extracted alanine, H is priority 4, so we need to look at the molecule
with the H atom pointing into the paper, like this.

3 Mentally move from substituent priority 1 to 2 to 3. If you are moving in a clockwise manner,
assign the label R to the chiral centre; if you are moving in an anticlockwise manner, assign the
label S to the chiral centre.

A good way of visualizing this is to imagine turning a steering wheel in the direction of the
numbering. If you are turning your car to the right, you have R; if you are turning to the left you
have S. For our molecule of natural alanine, if we move from NH2 (1) to CO2H (2) to CH3 (3)
we’re going anticlockwise (turning to the left), so we call this enantiomer (S)-alanine.

You can try working the other way, from the configurational label to the structure. Take lactic
acid as an example. Lactic acid is produced by bacterial action on milk; it’s also produced in your
muscles when they have to work with an insufficient supply of oxygen, such as during bursts of vigorous exercise. Lactic acid produced by fermentation is often racemic, though certain species of bacteria produce solely (R)-lactic acid. On the other hand, lactic acid produced by anaerobic respiration in muscles has the S configuration.